PALEOECOLOGY AND PALEOCLIMATOLOGY OF AN EARLY JURASSIC (TOARCIAN) VOLCANIC LAKE SYSTEM, ANTARCTICA: IMPLICATIONS FOR THE ROLE OF FERRAR MAGMATIC INTRUSION ON THE TOARCIAN CARBON CYCLE

The emplacement of the Chon Aike–Ferrar–Karoo large igneous province (LIP) through Paleozoic and Mesozoic coal is one of the proposed drivers of carbon cycle perturbations during the Early Jurassic. In Antarctica, fossiliferous lacustrine deposits, paleosols, and fluvial deposits are preserved within a sub-million year time interval during the emplacement of this LIP, and thus provide a direct record of the effects of LIP volcanism on high-latitude ecosystems. We integrate paleoecologic reconstructions of high-latitude environments with carbon and nitrogen isotopes of organic matter in chert and in late Paleozoic coal to assess the role of Ferrar magmatism on carbon cycle perturbations during the Early Jurassic. The fossils from this locality include: pollen cones (e.g., Cheirolepidiaceae) and a distinct pollen layer (Classopollis) that suggests salt-tolerance of some of the vegetation, and ferns. The sedimentary cherts display evidence of soft-sediment deformation, volume-loss, and are relatively sodium-rich in comparison to other abiogenic cherts. We interpret these cherts as similar in origin to some modern abiogenic chert– Magadi-type chert. The presence of both lacustrine Magadi-type chert and limestone provides a way to reconcile these disparate paleoenvironmental interpretations in that Magadi-type cherts can only form under very specific water chemistry characterized by high pH (>9.5), high ionic strength, and concentrations of dissolved silicon, conditions that may have supported salt-tolerant vegetation. In turn we hypothesize that lacustrine limestone is a record of relatively wetter periods, higher riverine flux of freshwater, and pH<9.5. Carbon isotope geochemistry reveals surprisingly negative δ¹³C values of vascular plant-associated organic matter analyzed and persistent isotopic shifts suggesting seasonal turnover of the lake system. The data suggests very negative δ¹³C values of paleoatmospheric CO₂, as opposed to low water stress, consistent with coincident magmatic intrusion through late Paleozoic coal on Gondwana.